A radio access network allows entities to communicate with each other over a wireless, radio channel. For example, a user equipment (UE) is able to communicate with network elements such as a base station over the radio access network. When entities communicate with each other they must conform to standard protocols so that each entity can correctly interpret the data received from other entities.
Third Generation Partnership Project (3GPP) Long Term Evolution, referred to as LTE, is a standard for wireless communication of high-speed data over a radio access network which allows a user equipment (UE) such as a mobile phone to communicate over the network. 3GPP technical specification TS 36.323 defines the functionality of the Packet Data Convergence Protocol (PDCP) for use in LTE. In particular the 3GPP TS36.323 specification defines (i) a sequence number, (ii) a discard timer, and (iii) a reordering window for Packet Data Convergence Protocol (PDCP) in LTE. PDCP is one of the layers of the Radio Traffic Stack in the Universal Mobile Telecommunications System (UMTS) and performs Internet Protocol (IP) header compression and decompression, transfer of user data, maintenance of sequence numbers for Radio Bearers and ciphering/deciphering of data.
According to PDCP, a new sequence number (SN) is assigned to each PDCP Protocol Data Unit (PDU), or in other words to each IP packet, when that PDU (or “packet”) is transmitted. The SN is used to align a sending PDCP entity (or “transmitter”) and a receiving PDCP entity (or “receiver”). For example, the PDUs form a data stream and the sequence numbers can be used (with hyper frame numbers as described below) to indicate the position of each PDU in the data stream. In this way, the sequence numbers assigned to the PDUs allow both the transmitter and the receiver to correctly align each PDU within the data stream. This is important so that the receiver can receive and correctly decipher the PDUs of the data stream that are transmitted from the transmitter. According to the current 3GPP standard, in acknowledged mode (AM), the SN is a 12 bit value which takes values from 0 to 4095, such that there are 4096 distinct sequence numbers. The SN is increased for each PDCP PDU and regularly loops over from 4095 to 0 every 4096 PDCP PDUs. At every SN loop over (that is, every time the sequence number changes from 4095 to 0), a local hyper frame number (HFN) is incremented. Both the transmitter and the receiver keep track of a local HFN to ensure that the transmitter and receiver are aligned with each other in terms of the positions of PDUs within a data stream. The HFN and the SN define a PDU's position within the data stream. The SN is assigned to the PDUs and transmitted with the PDUs from the transmitter to the receiver. In contrast, the HFN is not transmitted with the PDUs and instead, both the transmitter and the receiver keep track of the HFN locally. The HFN and SN are some of the input parameters for the ciphering/deciphering process which is used to encrypt/decrypt the data on the radio path.
If the transmitting entity and the receiving entity become misaligned with respect to their HFNs then the receiving entity will encounter problems when trying to decipher the PDUs of the data stream received from the transmitting entity.